Conventionally, an electron tube which is used for detecting the presence or absence of a flame on the basis of ultraviolet rays emitted from a flame in a combustion furnace or the like has been known. The electron tube includes a sealed container in which predetermined gas is filled and sealed, an electrode supporting pin that penetrates through the sealed container, and two electrodes that are supported in parallel with each other by the electrode supporting pin within the sealed container. In the electron tube, when one electrode arranged to oppose the flame is irradiated with ultraviolet rays in a state where a predetermined voltage is applied across the electrodes through the electrode supporting pin, electrons are emitted from the one electrode due to the photoelectric effect and excited in succession one after another to cause an electron avalanche between the one electrode and the other electrode. Therefore, it is possible to detect the presence or absence of a flame by measuring a change in impedance between electrodes, a change in voltage between electrodes, and electric current flowing between electrodes. Various methods for detecting the presence or absence of a flame have been suggested.
In the related art, there has been suggested a method in which electric current flowing between electrodes is integrated and it is determined that a flame is present in a case where an integrated value is greater than or equal to a predetermined threshold value and a flame is absent in a case where the integrated value is less than the predetermined threshold value (for example, see Patent Document 1). However, in the method, since electric current flowing between the electrodes is integrated, time for integration is required even when extinguishing the flame. Therefore, time is required until extinguishment of flames is detected. As a result, it is difficult to rapidly perform detection of the presence or absence of the flame.
In order to solve the problems described above, a flame detecting device disclosed in Patent Document 2 includes an electron tube which is provided with a pair of electrodes and in which electrons are emitted between the electrodes when the electrodes are irradiated with ultraviolet rays, an applying portion that applies a periodically changing voltage between the electrodes, a detecting portion that detects a voltage waveform representing a temporal change of the voltage between the electrodes, and an determining portion that determines the presence or absence of a flame on the basis of the voltage waveform. As such, the flame detecting device disclosed in Patent Document 2 detects the voltage waveform representing the temporal change of the voltage between the electrodes provided in the electron tube and determines the presence or absence of flames on the basis of the voltage waveform and thus, the time for integration is not required. Therefore, Patent Document 2 is an invention which is capable of rapidly performing detection of the presence or absence of the flame.